1. Field of the Invention
This invention relates to heating systems for use in semiconductor processing equipment and in particular to a heating system for use in a chemical vapor deposition system for forming layers of materials on semiconductor wafers.
2. Description of Related Art
Equipment for the formation of layers, such as epitaxial silicon or polysilicon, on semiconductor wafers is well known. One of the problems in forming layers of materials on a semiconductor wafer is ensuring that the temperature of the semiconductor wafer is kept uniform across the wafer during the deposition process, i.e., during heat-up, processing, and cool-down. Since the deposition rate of a layer of material upon the wafer is dependent on the temperature of the wafer, any temperature variations between the center and edge of a wafer will undesirably result in the deposition of a layer of non-uniform thickness on the wafer. In addition, non-uniform temperatures during heat-up, processing, or cool-down can cause stress on the wafer, undesirably resulting in slip. Accordingly, it is important during the deposition of a layer of material on a silicon wafer to minimize temperature variations across the surface of the wafer.
Some earlier systems developed to achieve temperature uniformity across a wafer teach applying heat in a uniform manner across all portions of a wafer. However, since heat loss is typically greater at the edge of a wafer than at the center, such earlier systems may result in significant temperature differences between the center and outer portions of the wafer and, thus, may be largely ineffective in minimizing temperature gradients across the surface of a wafer.
Some more recent heating schemes employ lamp assemblies having heating lamps arranged in a plurality of independently controlled zones to allow differing amounts of heat to be applied to the outer and center portions of the wafer. In this manner, variations of heat loss on a wafer can be compensated by applying differing amounts of heat to various portions of the wafer in order to achieve greater temperature uniformity across the wafer. Such techniques are typically able to maintain temperature differences of less than 10.degree. C.
The heating lamps employed in such multiple-zone heating assemblies may result in several disadvantages. Each of these lamps, which is typically tungsten halogen, has a finite useful life and thus must be periodically replaced. The replacement of such lamps, as well as periodic adjustment of the lamps due to their aging, not only increases manufacturing costs but also decreases the throughput of the chemical vapor deposition (CVD) system.
Furthermore, the use of such heating lamps in a radiantly heated CVD system undesirably requires periodically cleaning the quartz walls of the reaction chamber to remove deposited material from the chamber walls. Recall that since the walls in a lamp heated CVD reaction chamber become heated, layers of material deposit not only on the wafer but also on the chamber walls. The resultant film formed on the chamber walls then absorbs some of the radiant energy emitted from the heating lamps and thereby locally increases the temperature of the chamber walls. As a result, layers of material deposit on the chamber walls at an increasing rate, thereby creating a "snowball" effect.
It is therefore critical, in a lamp heated CVD chamber, that the walls be kept meticulously clean. This requires that the walls be etched frequently, sometimes even after every run. Cleaning the walls typically takes between 2 to 4 minutes after depositing an epitaxial layer, and longer after depositing polysilicon. Since the entire cycle time in warm wall CVD systems may be between 5-10 minutes, cleaning the walls accounts for a significant portion of the cycle time and, therefore, severely reduces throughput of the CVD system.
Accordingly, it is desirable to provide a heating system for chemical vapor deposition which uniformly and precisely heats a semiconductor wafer without the above-mentioned deficiencies.